U.S. military aircraft engines were at first designated by the manufacturer’s designation. This system, or lack of one, was used until about 1926. After this, both services adopted a system that consisted of a cylinder arrangement letter; a number that approximated the cylinder displacement (rounded to the nearest 5); and a version number. Odd version numbers were for Army engines and even numbers for Navy engines. At first, both services used letters instead of numbers. There was no provision for anything other than piston engines at first. The piston engine system is still in use with minor modifications. X, Y, and Z prefixes were used as applicable in the same manner as aircraft. A suffix was sometimes used. W stood for water injected engines. A, B, C… suffixes were used to indicate modifications minor enough not to justify a new dash number. In a few cases, AN was used to denote an engine that met Army/Navy combined specifications.

Examples:

Piston Engine Cylinder Arrangements

H – Double Horizontally Opposed

I – Inverted V

L - 1 Row Inline

O- Horizontally Opposed

R - Radial

V – 2 Row Inline

W – 3 Row Inline

X – 4 Row Inline

In 1946 provision was added for jet and rocket engines. The jet series numbering inexplicably started at 30. The rocket series started at 1. Even numbers were used for Navy sponsored engines, and odd numbers were for Army or Air Force sponsored engines. The piston engine system continued as before. Manufacturer letters were used in the designation. A table of these is below.

Examples:

Jets and Rockets 1946-1968

J- Turbojet

LR- Liquid-Fuel Rocket

PJ – Pulsejet

RJ – Ramjet

SR- Solid-fuel Rocket

T - Turboprop

TF – Turbofan

In 1969, the system for designating jet-type engines was modified. Each of these series was restarted using 3 digits. 100 numbers were for Air Force sponsored Engines, 400 for Navy sponsored engines, and 700 for Army sponsored engines. Dash numbers were assigned in a similar manner. Most new piston engines are designated by the manufacturer’s designation. Most of these are O- types and the manufacturers use a system similar to the old Army/Navy piston engine system.